The research team led by Dr Geoffrey Woods, from the Cambridge Institute for Medical Research, UK, found that a rare inherited gene mutation stops pain-sensing neurons from functioning.
Dr Woods was on a trip in Pakistan when he heard the story of a local boy who could pass knives through his arms without feeling any pain and decided to investigate. Although the boy had passed away, Dr Woods found six other people who had also never experienced pain, including the boy's mother.
The six were from three different families but they were all from northern Pakistan and all were members of the same Qureshi birdari clan as the boy.
Dr Woods said: "This paper shows that rare diseases can still be of great importance, because of the insights they give into biological and developmental processes."
Pain is felt when electrical impulses from nociceptive neurons travel to the brain. The original nerve stimulation produces small changes in voltage across the membrane that surrounds the nerve cell. A voltage-gated sodium channel protein called Nav1.7 amplifies these signals and when the potential difference reaches a threshold level, the neuron fires.
The researchers found each of the three families had different mutations in the SCN9A gene. However, all of the mutations are expected to cause prematurely truncated proteins or mRNA decay and hence loss of Nav1.7 function. This, in turn, causes a total inability to feel pain.
Including Nav1.7, there are ten different sodium channel proteins (encoded by ten different genes) and they all have a high degree of similarity in their amino acid sequences and a highly conserved structure.
Several are found in the nociceptors and it is therefore suprising that even when Nav1.7 is not produced, one of the other isoforms doesn't support its function.
Nav1.7 is also found in non pain related neurons yet the people studied by Woods could still perceive things like touch, pressure and the position of their limbs.
The research team also speculate that defects in the SCN9A gene could also be responsible for differing pain thresholds.
Some types of pain do not stem from nerve stimulus, such as that stemming from nerve or tissue damage. This neuropathic or inflammatory pain is unresponsive to current treatments but a drug that prevents Nav1.7 from being expressed might work. Such a drug could also replace local anaesthetics as it could have fewer side effects.
Pain is not necessarily a bad thing though; several of the people studied by Woods had bitten off the tips of their tongues in infancy and the boy who inspired the research died jumping off a roof.
"The work of Geoff Woods and his team has provided us with an exciting new target for pain killing drugs - potentially this is as important as the identification of the morphine receptors," said Dr John Wood, University College London, who collaborated in the study.
"It is fascinating that this same gene, when mutated to encode a hyperactive channel, has also been found to contribute to ongoing pain in some heritable human disorders."